Commercially available filters remove particulates from many different types of liquids and gases. Particulates are removed from machining lubricants, coolants, engine oil and so forth. Particulates are removed from stack gases, from air cycled through heating and air conditioning systems, and from air before the air is combined with a fuel for combustion in engines.
Filler structures are characterized by an inverse relationship between filter efficiency and porosity. A filter structure having an improved ratio of filter efficiency to porosity would beneficially result in a higher flow rate at a given filtration efficiency and among other things, reduce power consumption compared to traditional filter structures.
As illustrated by U.S. Pat. No. 5,057,368, high loft filters of high efficiency may be formed from trilobal or quadrilobal fiber. The fiber polymer may be a blend of polyester and polyolefin.
As exemplified by Filtration News, pp. FN 32-33 (Sept./Oct. 1989), wet laid, filter media are known. Compared to spunbonded and dry laid products, improved uniformity has been observed for the wet laid products. The relationship between fiber diameter, filter efficiency and porosity for the homofil webs investigated therein, is illustrated in the graph lines (Homofil Webs) of FIGS. 1 to 3 attached hereto.
Generally speaking, high efficiency filters formed from microfiber, typically 0.25 micron microglass, are relatively expensive. A less costly, high efficiency filter would be advantageous. Also, it would be beneficial to be able to control filter pore size in a cost-efficient manner with commercially available materials.
Dry processes of making nonwoven fabrics from a blend of bicomponent fiber and natural or synthetic fiber are exemplified by European Patent Application No. 0 070 164 to Fakete et al, which discloses a low density, thermobonded, nonwoven fabric comprising staple length polyester/polyethylene bicomponent fiber and short length natural cellulose fiber. The thermal bonding is at a temperature sufficient to fuse the polyethylene component without fusing the polyester component, while the web is maintained under little or no compression.
As illustrated by U.S. Pat. No. No. 5,167,765 and European Patent Application No. 0 311 860, heat-bonded, wet laid structures may be made from bicomponent fiber and optionally an additional fiber type. An exemplary bicomponent fiber for this purpose, is available under the registered trade mark CELBOND.RTM. from Hoechst Celanese Corporation of Charlotte, N.C. This bicomponent fiber consists of a polyester web-forming core and a low melting sheath, and is also available as crimped fiber for dry lay applications.
As indicated, an improved ratio of filtration efficiency to porosity and improved control of porosity are desirable. Accordingly, there is a need for an improved filtration structure characterized by a higher flow rate at a given filtration efficiency. Moreover, there is a need for a filtration structure that may be customized to a wide range of filtration efficiencies/porosities at reasonable cost.